<!doctype html>
<html lang="zh-CN">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="https://cdn.bootcss.com/jquery/3.2.1/jquery.min.js"></script>
    <script src="https://cdn.bootcss.com/bootstrap/4.0.0-alpha.6/js/bootstrap.min.js"></script>
    <link rel="stylesheet" href="css/bootstrap.min.css">
    <style>
        .row{
            margin-bottom: 20px;
        }
        /*.row .row{*/
        /*margin-top: 10px;*/
        /*margin-bottom: 0px;*/
        /*}*/
        [class*="col-"]{
            padding-top: 15px;
            padding-bottom: 15px;
            background-color: #eee;
            background-color: rgba(86,61,124,.15);
            border: 1px solid #dddddd;
            border: 1px solid rgba(86,61,124,.2);
        }
    </style>
</head>
<body>
<div class="container-fluid">
    <div class="row">
        <div class="col-sm-9">
            Level 1: .col-sm-9
            <div class="row">
                <div class="col-xs-8 col-sm-6">
                    Level 2: .col-xs-8 .col-sm-6
                </div>
                <div class="col-xs-4 col-sm-6">
                    Level 2: .col-xs-4 .col-sm-6
                </div>
            </div>
            <!--被嵌套的行（row）所包含的列（column）的个数不能超过12（其实，没有要求你必须占满12列）-->
        </div>
    </div>
</div>

</body>
</html>